1. Field of the Invention
This invention relates to a method of producing .beta.-sialon fine powder.
2. Description of the Prior Art
.beta.-Sialon represented by the general formula of Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z (0.ltoreq.z.ltoreq.4.2) is a solid solution of .beta.-Si.sub.3 N.sub.4 of which the Si position is substituted with Al and the N position is substituted with O. Sintered bodies of .beta.-sialon are excellent in strength, heat resistance, corrosion resistance and resistance to oxidation, and are utilized for cutting tools, parts of engines, parts of gas turbines and the like.
For the synthesis of .beta.-sialon powder, various methods are known, such as pyrolyzing kaolinite occurring in nature in an ammonia gas atmosphere (Journal of Material Sceince, 11, 1972-1974, 1976), pyrolyzing a coprecipitation product of SiO.sub.2 and Al(OH).sub.3 in an ammonia gas atomosphere (Am. Ceram. Soc. Bull., 58, 191-193, 1979), adding aluminum metal to a mineral such as shirasu or quartz sand and reducing the mixture in an inactive atmosphere (Yogyo Kyokai-shi, 88, 361-367, 1980), adding C to a SiO.sub.2 -Al.sub.2 O.sub.3 mixture and burning in a nitrogen gas stream (Yogyo Kyokai-shi, 95, 570-574, 1987), and mixing amorphous Si.sub.3 N.sub.4 with Al.sub.2 O.sub.3, AlN and burning in a nitrogen gas stream (Japanese Patent KOKAI No. 62-72507).
Among the above methods, in the case of using natural materials such as kaolinite, shirasu and quartz sand, the synthetic powder is liable to be contaminated with impurities, while in the case of burning the coprecipitation product of SiO.sub.2 and Al(OH).sub.3, the mixture of SiO.sub.2 -Al.sub.2 O.sub.3 and the mixture of amorphous Si.sub.3 N.sub.4 with Al.sub.2 O.sub.3, AlN, the reaction time is long, and unreacted materials remain in the reaction product, because the reaction occurs between solid phases. Moreover, the substitution of the Si position with Al and that of the N position with O are insufficient due to the solid phase reaction, and the bending strength of the particles in the sintered body produced from the above conventional powder is also insufficient. This causes reduced strength and resistance to oxidation of the sintered body.